The present invention relates to a hydraulic system for the traction of a hybrid vehicle, and more specifically a hybrid vehicle with a hydraulic circuit.
French Patent No. 2973302, hereby incorporated by reference, discloses a transmission for a hydraulic hybrid vehicle, having a planetary gear set having three elements that are connected to a thermal engine, a hydraulic pump, and a differential output for driving the motor wheels. The transmission further includes a hydraulic system that works as a motor or pump, which connects to the differential output with several gear ratios.
The hydraulic system comprise a low-pressure circuit and a high-pressure circuit each having of a pressure accumulator for storing energy. The low pressure accumulator maintains a minimum pressure to avoid cavitation of devices. The stored pressures are then recharged for applying a motor torque to the wheels.
Thus, different operation modes are achieved including a “pulse” mode with traction of the vehicle solely by the hydraulic system, the thermal motor being stopped, and a “bypass” mode with traction of the vehicle by the thermal motor which delivers, via the planetary gear, torque to both the differential output, and the pump supplying hydraulic power. In the bypass mode, according to the speed of rotation of the pump, there are an infinite number of gear ratios between the engine and the drive wheels. An additional traction torque provided by the hydraulic system can also be added in this mode.
In a “short connection” mode, the pump is off and the combustion engine delivers a high torque to the drive wheels of the planetary gear set forming a speed reduction gear. In a “long connection” mode, the combustion engine delivers a lower torque from the drive wheels through the planetary gear set, which is blocked. Further in a “braking” mode, the hydraulic system acting on the pump delivers a braking torque to the vehicle, recharging the high-pressure reservoirs.
In addition, the bypass mode can isolate the two circuits of the high-pressure accumulator to work with a very high pressure, above the acceptable pressure for the accumulator, between the pump delivering the flow directly to the hydraulic system operating as a motor. One then obtains with these pumps greater torques and power.
A problem with this type of hydraulic system with devices that can be isolated from a pressure accumulator to work together, is the need to ensure safety in case of accidental overpressure on the high pressure circuit.
Generally speaking, it is necessary for the hydraulic pump of a hybrid vehicle connected to a transmission with hydraulic devices, including a transmission of the type described above, to meet the special needs important for automotive vehicles for production on a large scale, including reduced weight, good compactness, excellent efficiency, high reliability and low cost. An optimized circuit therefore must be provided, which tends to reduce the number of components to enhance these various criteria. In particular, the place in a vehicle is limited, and its weight has a direct effect on energy consumption as well as on its autonomy. Furthermore, the hydraulic circuit must allow for easy maintenance, and must in all cases ensure safety in case of accident or fire.